Here now is a diagram
explaining the filtration
apparatus in the glomerulus.
Down the bottom
is a diagram illustrating a vessel, a
fenestrated capillary that's shown here in orange.
The kidney has fenestrated endothelium
in the glomerulus to enhance
the chance of filtration, to enhance the
ability for the plasma to leak out into
Bowman's capsule. And when it leaks out,
it passes through gaps between the podocyte
and a molecular sieve I'll describe in a
moment. Have a look at the podocyte.
It's colored yellow in the bottom diagram.
The podocyte is like an octopus wrapping
its tentacles all around the capillary.
And as you see here when it
wraps its processes around the capillary,
it creates filtration slits. And then
between the podocyte and the endothelium
is the basal lamina. It's a
combined basal lamina produced by both
the podocyte and the endothelial cells.
That's a very important structure too.
And you can see details of that
in a higher magnification or a higher
representation on the top part of the
diagram in the little square, or probably
rectangle. Now here's an electron micrograph
showing the details of that filtration
barrier. Let's orientate ourselves. Down the
bottom is the lumen of the capillary in
the glomerulus. It has got fenestrations
or little gaps in the wall. So the plasma
is going to move out through
those fenestrations. And then it confronts
this basal lamina, which I said
was produced by both the podocyte and
the endothelial cell. It's a fairly thick
layer. It's a molecular sieve.
It prevents certain substances,
molecules, from passing through it.
Most proteins, for instance, are too large
to get through that sieve. It's anionic so
it tends to repel negatively charged molecules.
And then further filtration is
achieved by substances passing through
this molecular sieve then through the
filtration slits created by those podocyte
processes, and there's also a very
very thin molecular diaphragm there. It also
participates in the filtration
process. Finally, the filtrate passes into
the luminal space or Bowman's space.
And that's the urinary space. And up on the
top, you can see part of the cell body of
one of these podocytes. So that's the
filtration apparatus, and filtration is
achieved by the hydrostatic pressure
created by the efferent arteriole having a
smaller diameter than the larger
afferent arteriole. And this pressure
here created across the glomerulus
achieves the pressure required to drive
that plasma filtrate through the
molecular sieve into Bowman's space.